Only use Gnus registry for formatting when registry is loaded
See bug #36903 * lisp/gnus/gnus-registry.el (gnus-registry-article-marks-to-chars): (gnus-registry-article-marks-to-names): In some circumstances it's possible for the user's summary line format spec to include registry-specific code, while the registry itself isn't actually loaded. Make sure the database is actually a hashtable before accessing it.